Photovoltaic solar cells essentially comprise a semi-conductor substrate of one conductivity type having a shallow P-N junction formed adjacent the front surface thereof The cells require electrical contacts (also called "electrodes") their front and rear sides in order to be able to obtain electrical current from the cells when they are exposed to solar radiation. The contact on the front of the solar cell is generally made in the form of a grid, comprising a plurality of narrow, elongate parallel fingers that extend in one direction, and at least one elongate bus that intersects the fingers at a right angle. The width, number and spacing of the fingers are arranged so as to expose an optimum area of the front surface of the cell to incident solar radiation.
In order to improve the conversion efficiency of the cells, a thin anti-reflection coating consisting of a material such as silicon nitride or an oxide of silicon or titanium is provided on the front sides of the cells. The anti-reflection coating may be applied before or after the electrode has been formed.
Widespread use of solar cells is desirable but has been handicapped by cost, reliability and efficiency factors. The efforts to provide efficient cells with long term reliability at an acceptable cost are complicated by the nature of the substrate and the interrelationship of the various process steps and parameters. Thus, for example, in an effort to reduce the cost of solar cells, it has been deemed desirable to use relatively low-cost polycrystalline ribbons or sheet material instead of single crystal silicon. Accordingly, efforts have been made to develop fabrication techniques whereby efficient and reliable solar cells may be produced from EFG-grown silicon substrates which are polycrystalline. However, EFG substrates tend to have surface irregularities and also may have silicon carbide particles on their surfaces. Therefore it is essential to have a method of producing electrical contacts that is not sensitive to surface irregularities in order to maximize cell yield and efficiency. The steps of forming the front and rear contacts are a critical part of the manufacturing process; the contacts must be capable of being formed relatively inexpensively without physically damaging the brittle EFG substrates and without any deterioration of the P-N junction or cell performance.
Various materials have been used as electrical contacts for photovoltaic solar cells, the most common being aluminum, silver, and nickel. A common arrangement with silicon solar cells is to make the rear contact of aluminum and the front contact of silver.
One prior art method of forming a grid electrode involves applying a conductive metal paste onto the front surface of the substrate in a clearly defined grid pattern, firing that paste so as to form a bonded ohmic electrical contact, and then applying an anti-reflection coating to the front surface of the solar cell. Another common procedure is to first form an anti-reflection coating on the front surface, then etch away portions of that coating so as to expose portions of the front surface of the substrate in a grid electrode pattern, and thereafter deposit or otherwise form the front contact on the front surface in the region where the anti-reflection coating has been etched away.
Still another approach is the so-called "fired through" method which consists of the following steps: (1) form an anti-reflection coating on the front surface of the solar cell, (2) apply a coating of a metal/glass frit ink or paste onto the anti-reflection coating in a predetermined pattern corresponding to the configuration of the desired grid electrode, and (3) fire the paste at a temperature and for a time sufficient to cause the metal/glass composition to dissolve the anti-reflection coating and form an ohmic contact with the underlying front surface of the substrate. The "fired through" method of forming contacts is illustrated by PCT Patent Application Publication WO 89/12321, published Dec. 14, 1989, based on U.S. application Ser. No. 205,304, filed Jun. 10, 1988 by Jack Hanoka for An Improved Method Of Fabricating Contacts For Solar Cells, now abandoned, and refiled as a continuation-in-part of U.S. application Ser. No. 07/607,883, filed Nov. 1, 1990, the concept of firing metal contacts through an anti-reflection dielectric coating also is disclosed in U.S. Pat. No. 4,737,197, issued to Y. Nagahara et al for "Solar Cell With Metal Paste Contact".
Both thin film and thick film techniques have been used for the formation of electrical contacts on solar cells. Thin film manufacturing techniques include vapor deposition, sputtering, and electroless plating. Thin film manufacturing techniques permit the formation of pure metal contacts, with the result that the contacts exhibit excellent electrical properties. However, thin film techniques are costly or have environmental problems.
Thick film technology involves using a suitable paste or viscous ink to form a relatively thick metal-containing film on a substrate, and then firing that film so as to make an electrically conductive layer that is strongly bonded to the substrate. Screen printing, pad printing and the so-called "direct write" technique are among the ways of forming thick film components on semiconductor substrates.
Representative of commercially available direct writing systems in use prior to this invention is the "Micropen" machine sold by Micropen, Inc. of Pittsford, N.Y., USA, which apparently is based on the writing apparatus described and illustrated in U.S. Pat. No. 4,485,387, issued Nov. 27, 1984 to Carl E. Drumheller, for "Inking System For Producing Circuit Patterns". While such systems are capable of writing grid patterns on cambered and uneven substrates, they are not satisfactory for forming grid contacts on solar cells on a production line basis for various reasons, including inability to maximize the aspect ratio of the fingers (the aspect ratio is the ratio of finger height to finger width), an insufficient reduction in the cost of solar cell manufacture, and high capital equipment cost due to the complexity of the equipment which includes computer controls and software designed to promote real-time dynamic pen position control. Still other limitations exist when forming contacts on poylcrystalline substrates that have surface irregularities such as ripples and bumps caused by silicon carbide particles, e.g., EFG-grown silicon substrates.
An essential consideration is the need for development of a contact-forming technique that can be integrated into a mass-production manufacturing process. An ideal contact-forming process is one that is continuous rather than a batch-type operation.
A long-standing goal of solar cell makers has been to achieve the ability to form front grid-type contacts with tall as well as narrow fingers. Making the fingers narrow is desirable to minimize light shadowing (i.e., to expose as much as possible of the front surface of the solar cell to solar radiation). Increasing the height of the fingers without a corresponding increase in finger width is desirable so that the current-carrying capacity of the fingers is maximized. Another long-standing goal has been to reduce the cost of manufacturing grid contacts. The latter goal involves the task of increasing the rate at which the contacts are formed. While the contacts may be formed in a single operation, the differences in the widths and functions of the bus bars and the finger elements makes it feasible to form the bus bars and fingers in two separate operations. Formation of suitable bus bars can be accomplished satisfactorily using various techniques, such as screen printing or pad printing. Accordingly formation of the finger elements is the primary factor limiting the rate at which the grid contacts can be formed, since it is essential that the fingers have a narrow but optimim width, without sacrificing electrode quality or reliability.